Interesting energy source...
 

Came across this while researching for my energy series...The Earth's Electrical Environment

Ever hear of Telluric Currents? Also known as Earth current is an electric current which moves underground or through the sea. Telluric currents come from both natural causes and human activity such as oil pipelines! They are extremely low frequency and travel over large areas at or near the surface of Earth. I can only imagine how it might mess up how whales communicate.

Doing a quick search, looks like over a hundred years ago folks were making batteries (power sources) using dissimilar metal plates to powere telegraph machines. There seemed to be issues with resistivity and reliability and consistancy but it sure does make you wonder if with the advent of better alloys and rare earths and capacitor and battery design that maybe this should be looked at again?

I shall include this in my volume on electro-magnetic energy sources...:)

I don't know the physics of it, but 35 years ago, I worked on a Tokamak at MIT, we had, then garbage can sized capacitors that could store about a farad of electricity (small by today's standards) but the capacitors were shorted with a wire because the Earth's magnetic field could put a pretty good jolt of electricity into them.

Not the earth's magnetic field:

Dielectric absorption is the name given to the effect by which a capacitor that has been charged for a long time discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage coming from time-delayed dipole discharging, a phenomenon that is also called dielectric relaxation, "soakage" or "battery action". For some dielectrics, such as many polymer films, the resulting voltage may be less than 1-2% of the original voltage, but it can be as much as 15% for electrolytic capacitors. The voltage at the terminals generated by the dielectric absorption may possibly cause problems in the function of an electronic circuit or can be a safety risk to personnel. In order to prevent shocks most very large capacitors are shipped with shorting wires that need to be removed before they are used.